That is the Oxford Dictionaries Word of the Year 2016. According
to the Oxford Dictionaries website (link here),
it is defined as "relating to, or denoting, circumstances in which
objective facts are less influential in shaping public opinion
than appeals to emotion as and personal belief". It is used as an
adjective. For example, the post-truth world that we live in
today can be attributed to the speed at which some opinions,
half-truths and falsehoods get shared and the willingness of the recipients
(people) to believe, embrace and re-distribute them.